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Astronomers Take a Closer Look at Comet ISON

Astronomers Take a Closer Look at Comet ISON

Astronomers from the University of Maryland, along with Lowell Observatory, have had the rare opportunity to observe comet ISON in close detail, which may become one of the most dazzling in decades when it rounds the sun later this year.

Using images acquired over the last two months from NASA's Swift satellite's Ultraviolet/Optical Telescope (UVOT), the team has been able to make initial estimates of the comet's water and dust production and used them to infer the size of its icy nucleus.

"Comet ISON has the potential to be among the brightest comets of the last 50 years, which gives us a rare opportunity to observe its changes in great detail and over an extended period," says lead investigator Dennis Bodewits, an astronomer at UMD.

In late February, a team of comet experts initiated the Comet ISON Observing Campaign (CIOC) to assist ground- and space-based facilities in obtaining the most scientifically useful data.

Like all comets, ISON is a clump of frozen gases mixed with dust. Often described as "dirty snowballs," comets emit gas and dust whenever they venture near enough to the sun that the icy material transforms from a solid to gas, a process called sublimation. Jets powered by sublimating ice also release dust, which reflects sunlight and brightens the comet. Typically, a comet's water content remains frozen until it comes within about three times Earth's distance to the sun.

In January, UVOT observations found that ISON was shedding a significant amount of dust, but a much smaller amount of water. "The mismatch we detect between the amount of dust and water produced tells us that ISON's water sublimation is not yet powering its jets because the comet is still too far from the sun," Bodewits says. "Other more volatile materials, such as carbon dioxide or carbon monoxide ice, evaporate at greater distances and are now fueling ISON's activity."

Similar levels of activity were observed in February, and the team plans to do additional UVOT observations.

While the water and dust production rates are relatively uncertain because of the comet's faintness, they can be used to estimate the size of ISON's icy body, which they've found is a typical size for a comet.

An important question is whether ISON will continue to brighten at the same pace once water evaporation becomes the dominant source for its jets.

"It looks promising, but that's all we can say for sure now," said Matthew Knight, an astronomer at Lowell Observatory in Flagstaff, Ariz., and a member of the Swift and CIOC teams. "Past comets have failed to live up to expectations once they reached the inner solar system, and only observations over the next few months will improve our knowledge of how ISON will perform."

Based on ISON's orbit, astronomers think the comet is making its first-ever trip through the inner solar system. The first of several intriguing observing opportunities occurs on Oct. 1, when the inbound comet passes about 6.7 million miles from Mars. Fifty-eight days later, on Nov. 28, ISON will make a sweltering passage around the sun. Around this time, the comet may become bright enough to glimpse just by holding up a hand to block the sun's glare. Following ISON's solar encounter, the comet will depart the sun and move toward Earth, appearing in evening twilight through December. Whether we'll look back on ISON as a "comet of the century" or as an overhyped cosmic dud remains to be seen, but astronomers are planning to learn the most they can about this unusual visitor no matter what happens.